Ion source having an annular permanent magnet



Oct. 29, 1968 c. w. ELENGA ET AL 3,408,526

ION SOURCE HAVING AN ANNULAR PERMANENT MAGNET 2 Sheets-Sheet 1 Filed April 18, 1966 FIG? AGEN

0d. 29, 1968 c, w, ELENGA ET AL 3,408,526

ION SOURCE HAVING AN ANNULAR PERMANENT MAGNET Filed April 18, 1966 2 Sheets-Sheet 2 1200 GAUSS CORNELI S WE IZEW TT R IFENSCH El ER h dAR mu s HMSCHENK LAARS 2M FAA/ 1M United States Patent ABSTRACT OF THE DISCLOSURE An ion source for a neutron generator and ion accelerators employing two spaced apart substantially fiat plates, one of which is apertured to permit ions to exit, constituting a cathode and a hollow, tubular anode therebetween. A permanent magnet positioned behind the non- I apertured plate provides an axial field while an annular magnet, coaxial with the exit aperture, providing a predominant magnetic field in the axial direction, aids the field of the first magnet. A soft iron envelope surrounds the assembly and completes the magnetic circuit.

The invention relates to an ion source in which ions are formed in a discharge between two substantially fiat plates as a cathode and a cylindrical anode surrounding the space between the two cathode plates one of which plates has an exit aperture for the ions. An axial magnetic field is produced in the ion source by means of a block of permanent magnetic material, which joins the non-perforated cathode plate. With the exception of the exit aperture for the ions a weak magnetic circuit surrounds the ion source completely.

Ion sources of this kind are frequently employed in neutron generators and other ion accelerators. The advantage of such an ion source is that the operational pressure may be low, i.e., to 10 mm. Hg. Furthermore, one voltage source of a few kilovolts is required, While the ion source current need not be very high 'as compared with the resultant ion beam. Because of the closed magnetic circuit, the magnetic stray field outside the ion source is very small, which is required with respect to the acceleration of the ions in the neutron generator or another apparatus in which the ion source is employed. Due to the weak stray field accelerations up to 200 kv. are permissible in one step.

Known ion sources have, however, the disadvantage that it is difiicult to obtain a beam current of more than a few milliamperes with the conventional dimensions of the ion source, i.e., a length of a few centimeters and a diameter of the electrodes of a few centimeters.

It is a principal object of the invention to provide an improved ion source so that higher ion currents can be derived from the source.

In accordance with the invention, the ion source forms ions in a discharge between two substantially fiat plates forming the cathode and a cylindrical anode surrounding the space between the two cathode plates, one of which plates has an exit aperture for the ions. An axial magnetic field is produced by means of a block of permanent magnetic material, which joins the non-perforated cathode plate and, with the exception of the exit aperture for the ions, a weak-magnetic circuit surrounds the ion source completely. An annular permanent magnet is arranged on the inner side of a soft magnetic envelope coaxial with the exit aperture for the ions so that the magnetic field of the block of permanent magnetic material is amplified.

In the absence of the permanent magnet ring according v 3,408,526 Patented Oct. 29, 1968 to the invention the strength of the magnetic field near the exit aperture-is very slight, even if this aperture is fairly small. This low strength of the magnetic field results in that the formation of ions at that area is small, so that only a weak ion beam can emanate. Because of the presence of the permanent magnet ring according to the invention the strength of the magnetic field is considerably larger and, especially concentrated more along the axis; therefore, a stronger ion beam can be derived from the discharge. The presence of the permanent magnet ring also permits the enlargement of the exit aperture, which was, in general, from less than one-fifth of the anode diameter, to one-third to one-half of the anode diameter. The diameter of the hole in the permanent magnetic ring is preferably chosen not to exceed the exit aperture in the soft-magnetic envelope.

a The direction of magnetization of the permanent magnet ring may be axial or radial, provided the magnetic field amplified by the block of permanent magnetic material and the parts of the soft-magnetic envelope operating as pole shoes are sufficiently heavy for minimizing the stray field in the acceleration space.

The invention will be described more fully with reference to the drawing, in which FIG. 1 shows a neutron generator including an ion source according to the invention,

FIG. 2 shows on an enlarged scale the ion source thereof and FIG. 3 shows graphically the magnetic field of the ion source according to the invention.

The neutron generator shown in FIG. 1 comprises a glass cylinder 1, one end of which accommodates a tumbler-shaped acceleration electrode 2, in which the target plate 3 is arranged while at the other end of the glass cylinder 1 a metal cylinder 4 is fastened for supporting the ion source by means of stay rods. The envelope is filled With deuterium-tritium mixture the pressure of which is controlled by a hydrogen replenisher 14, de scribed more fully in US. Patent 3,240,790.

The ion source comprises a soft-iron envelope 5 with a rounded-off and perforated upper side 6 and a fiat bottom 7. A block of permanent magnet material 8 bears on the bottom and is coated on the discharge side with a layer of molybdenum 9. On the inner side of the pole shoe 6 a ring 10 of permanent magnet material is magnetized in the same direction as the block 8 While on the discharge side a thin molybdenum plate 11 is arranged. A discharge occurs between plates 9 and 11, which serve as cathodes, the discharge path being surrounded by anode 12. Block 8 has a depression 13 of about one-third of the diameter, the depth being approximately equal to the diameter.

In FIG. 3 the full line A indicates the course of the magnetic field in the ion source shown in FIG. 2 on an enlarged scale; it will be apparent that a small distance from the molybdenum plate 11 the magnetic field has a value of a few hundred gauss. The broken line B indicates the variation of the magnetic field if the ring 10 consists of soft-iron instead of permanent magnet material.

For known neutron generators having an anode diameter of 20 mm. and an exit aperture of 4 to 5 mm. for the ion beam the beam current is 0.1 ma. with a current in the ion source of 0.3 ma. With an acceleration voltage of kv. the neutron yield is 10 n./sec. The pressure is 10* mm. Hg. In pulsatory operation with a maximum pulse time of 10% the neutron yield during the pulses is 10 n./sec. with a beam current of 1 ma. and an ion source current of 3 ma. The pressure is then about l0 mm. Hg. The beam current is corrected for the secondary electron flow.

In a neutron generator comprising an ion source according to the invention, having anode diameter also of 20 current and the beam"curr'ent "are" mm. and a diameter of the hole of 10 mm. the ion source -0.2 a. and a neutron yield of 5x10 n./sec. could be obtained. The pressure then was 2 1() mm. Hg.

While the invention has been described with reference to a particular embodiment, other modifications will be apparent to those skilled in this art without departing from the spirit and scope of the invention which is defined in the appended claims.

What is claimed is:

1. An ion source comprising an envelope of magnetic material having an aperture therein for the passage of an ion beam, a pair of substantially flat, spaced plates within the envelope constituting a cathode, one of said plates having an aperture therein aligned with the aperture in the envelope, a hollow cylindrical anode surrounding the path of the ion beam formed between the cathode plates,

71119.. and lisiriafi a permanent magnet positioned adjacent the non-apertnred -cathode plate 'forproducing an axial magnetic field, and an annular permanent magnet providing a predominant magnetic field in the axial direction surrounding the aperture in the other cathode plate for amplifying the axial magnetic field.

2. An ion source as claimed in claim 1, in which the annular permanent magnet is magnetized axially.

I. Anion source as claimed in claim 1, in which the .diameter of the aperture in the perforated cathode plate is one-third to one-half of the diameter of the anode.

- References Cited UNITED STATES PATENTS 3,020,408 2/1962 Martin 313-61 3,082,326 3/1963 Arnold 31361 3,216,652 -1 1/1965 Knauer 3l37X FOREIGN PATENTS 972,191 10/1964 Great Britain.

JAMES W. LAWRENCE, Primary Examiner. S. D. SCHLOSSER, Assistant Examiner. 

